y varies inversely as x means:

y = k/x, for some constant k

lets find k.

y = 18 when x = 2 that is

18 = k/2 multiply both sides by 2

k = 2*18

k = 36

y = 36/x

when x = 10 we have

y = 36/10

y = 18/5

y = 3 3/5

or

y = 3.6.